Evaluation of the bulk water dynamics inside HeLa cells using terahertz time-domain attenuated total reflection spectroscopy

In spite of intensive efforts to elucidate the cellular functions and activities of biological macromolecules, such as DNA and proteins, the intermediary role of intracellular water in these activities is still cloaked in mystery. This is because bulk water dynamics, which occur on sub-picosecond and picosecond timescales, cannot be directly probed by conventional experimental techniques. With a periodic electric field on a sub-picosecond or picosecond timescale, terahertz waves are selectively sensitive to the dynamics of the hydrogen bond network associated with the bulk water. We use THz TD-ATR measurements, combined with a two-interface model, to determine the complex dielectric constant of a living cell monolayer, and thus characterize their intracellular water dynamics. Further interrogation was undertaken by decomposing the complex dielectric constant into its slow relaxation, fast relaxation and intermolecular stretching vibration components. The results indicated that intracellular water has a fragile hydrogen bond network, releasing water molecules from the network. Additionally, tetrahedrally coordinated water in the cell interior is structurally disordered compared to the extracellular water.